In recent years, the regulations on exhaust gases from engines have become stricter. To deal with these regulations, engine manufacturers and other makers have been trying in particular to reduce particulate matter (PM) and nitrogen oxides (NOx) contained in exhaust gases. A large number of advanced combustion control techniques have been developed. On the other hand, there have been developed techniques for performing the processes of capturing and purifying particulate matter (PM) and nitrogen oxides (NOx) by installing an exhaust gas after-treatment device such as DPF (diesel particulate filter) and a urea SCR system interposingly between the engine and the muffler. These techniques are combined as needed with advanced combustion control techniques to address the exhaust gas regulations that are getting stricter all the time.
However, the above-mentioned exhaust gas after-treatment devices such as the DPF (diesel particulate filter) and urea SCR system were not originally attached to the engine system. Usually, these exhaust gas after-treatment devices are complex in structure and use expensive materials. For example, the catalyst used in the PDF is platinum. Furthermore, the urea SCR system needs to be provided with a tank for storing urea and a urea injecting device. For these reasons, engine systems equipped with the exhaust gas after-treatment device can be considerably more costly than engine-only systems.
Meanwhile, in the field of hydraulic working machine such as hydraulic shovels, there have been proposed and developed in recent years hybrid-driven hydraulic working machines equipped with an assist electric motor driven by an engine as well as by an electric storage device such as batteries acting as a drive source.
For example, the construction machine (hydraulic working machine) proposed in Patent Document 1 is equipped with an engine-driven electric motor so that excess engine output is saved as electric energy for energy conservation. When engine output is insufficient, the saved electric energy is released to drive the electric motor so as to maintain required pump absorption torque. Patent Document 1 explains that this structure makes it possible to adopt a small engine having rated output equivalent to the average horsepower necessary for the construction machine to do its work and that fuel efficiency can be improved and exhaust CO2 emissions can be reduced accordingly.
The working machine proposed in FIG. 6 of Patent Document 2 is structured to generate hydraulic pressure with a hydraulic pump driven by an engine and an electric motor. The rate of increase in engine output is set to a predetermined value. An upper limit of engine output obtained from the predetermined rate of increase is compared with the demanded power acquired from the hydraulic output demanded of the hydraulic pump. If the comparison shows that the demanded power exceeds the upper limit of engine output, the exceeding output is compensated for by electric motor output. Patent Document 2 explains that even if hydraulic load is raised suddenly, this structure performs control to prevent engine load from getting abruptly raised thus keeping the operating conditions of the engine within an appropriate range and that the drop of combustion efficiency, generation of black exhaust, and engine shutdown can be avoided accordingly.